Fix another join problem. Don't use a TALLOC_CTX before it has been

Fix another join problem.  Don't use a TALLOC_CTX before it has been
initialized.

Also split out the oldstyle join into a new fn, allowing us to call it
with no failure message from net rpc join, but displaying a failure message
when used with net rpc oldjoin.
(This used to be commit cab0a4c4d5c7bf9d89697bf1d351eafbd00d7fd2)